Many conventional data transmission systems monitor the synchronization of received data frames to verify that the received data is in synchronization with a "superframe" that contains a plurality of multibit "frames". The synchronization monitor may use a method which is too ready to declare loss of synchronization, and is also less ready than is absolutely necessary to declare reestablishment of synchronization (the loss of synchronization forces a total loss of data traffic). A data transmission system that refuses to detect loss of synchronization until the worst possible operating conditions will work best with error correction.
Many conventional data transmission systems use some type of error correction (EC) encoding to send data from transmitter to a receiver. For example, ITU Recommendation G.742, which governs E1 and E2 data transmissions, specifies that each multibit "frame" of E2 data contains one "justification control flag" formed of three redundant justification control bits (also called "stuff bits") per each of the four multiplexed E1 "tributaries." These justification bits are used by the data transmission system to control the justification of received data. ITU Recommendation G.742 is herein incorporated by reference.
As is well-known in the art, three justification control bits per justification control flag allows one error in one justification control bit to be corrected. In this conventional method (majority decision), errors in two or more of the justification control bits cause the justification control flag value to be incorrect.
One type of error in the multibit E2 frame, an error in the justification control flag for an E1 tributary, causes that E1 tributary to experience a synchronization loss or "pattern slip."
When the frame is modified (by adding two-bit EC to the frame) so that the error threshold begins at three errors per frame, the uncorrected E2 frame bit error rate ("E2 BER") level at which pattern slips occur changes very little, although the E1 tributary bit error rate ("E1 BER") is actually greatly improved a that E2 BER level by the two-bit EC. The rate of pattern slips is therefore greatly increased with respect to the E1 BER.
A way of improving the EC of the justification control flag alone, rather than pay the cost of improving the EC of the frame as a whole, and an improved algorithm for superframe synchronization, which together greatly improve the synchronization loss level of the system with only a very slight decrease in data information efficiency, are needed.